colour vision deficiency.
The ability to see and distinguish the
different parts of the colour spectrum,
which consists of electromagnetic radi-
ation (energy waves) with a range of
wavelengths between about 400 and
700 nanometres (millionths of a milli-
metre). Different wavelengths trigger
nerve signals in the
sensitive layer of cells at the back of the
eye); these signals pass to the brain and
are interpreted as violet, indigo, blue,
green, yellow, orange, and red.
As light falls on the retina, it strikes
cones. The rods can detect all visible
light, but only the cones can distin-
guish colour. There are three types of
cone: red-sensitive, blue-sensitive, and
green-sensitive. Each of these types of
cone responds more strongly to a par-
ticular part of the light spectrum. The
cones are most concentrated in a cen-
tral area of the retina called the
for this reason, colour vision is most
accurate for objects that are viewed
Light, consisting ofradiation ofvarious wavelengths, is
focused on the retina. At the back ofthe retina, light-sensitive
rod and cone cells are stimulated to emit electrical impulses.
The impulses then travel through cells called bipolar cells to a
layer called the ganglion cells, where some initial processing
occurs, before passing to the brain via the optic nerve.
Location ofcolour-sensitive cells
The rods and cones are located at the back of
the retina. Behind them is a darkly pigmented
layer of cells, which reduces light scattering.
Colour vision depends on the cones, which
are concentrated in an area called the fovea.
— — — — —
Colour response of cones (right)
There are three classes of cone. One ofthese
(red-sensitive) responds best to light of long
wavelengths; another (blue-sensitive) to light
of short wavelengths; and the third (green-
sensitive) to intermediate wavelengths.
Response to white light
White light consists of a mixture
of all wavelengths (colours),
and ittherefore stimulates all
three classes of cone to signal
equally. This pattern of
response is interpreted as
whiteness bythe brain.
Response to red light
Light with a long wavelength (red
light) produces a strong response
from red-sensitive cones, a weak
cones, and an intermediate
response from green-sensitive
cones. This pattern ofsignalling
is interpreted as the colour red by